Oh and if you want info on weekly spills (at least the one's that get reported) there's a site that keep tabs on that sort of thing. http://healthyrivers.org

Actually 860 Billion is totally accurate. The EPA estimate of 850 billion gallons, as pointed out by Alex, is just Combined Sewer Overflows (CSOs). CSOs occur anytime it rains in cities like Boston, NYC, DC, Cleveland, Milwaukee, Baltimore and Portland Oregon just to name a few. The additional estimated 10 billion gallons come "accidents" (pump failures, power outages at sewer treatment plants, ruptured sewer lines, etc.) and Sanitary Sewer Overflows SSOs). While these are far less common they are real nasty and worse than CSOs since the sewage is not diluted with stormwater.